1. Field of the Invention
The present invention relates generally to a mobile communication system that transmits packet data over an uplink. More particularly, the present invention relates to a method and apparatus for controlling a data rate of user equipments in a mobile communication system.
2. Description of the Related Art
In Wideband Code Division Multiple Access (WCDMA) communication systems, Enhanced Uplink Dedicated Channels (E-DCHs) are used. The E-DCH has been aiming to increase packet transmission performance through an introduction of new technology in uplink communications in the WCDMA communication system.
The newly introduced technology can include Node-B based scheduling as well as Adaptive Modulation and Coding (AMC) and Hybrid Automatic Retransmission Request (HARQ) used in the existing High Speed Downlink Packet Access (HSDPA).
FIG. 1 is a basic conceptual diagram which shows where E-DCH may be used.
Referring to FIG. 1, the WCDMA communication system includes a Node B 100 supporting E-DCH and a plurality of user equipments (UEs) 101, 102, 103 and 104 receiving the E-DCH.
Node B 100 monitors channel conditions and buffer states of the UEs 101 to 104 supporting E-DCH, generates a scheduling command depending on the monitoring results, and transmits the scheduling command to each of the UEs. The UEs 101 to 104 determine a maximum allowable data rate of uplink E-DCH data according to the scheduling command, and transmit the data at the determined data rate.
However, in the uplink, the uplink signals transmitted by different UEs are not synchronized (or not orthogonal) with each other, thus serving as interference to each other. As a result, an increase in a number of uplink signals received at Node B increases the interference to an uplink signal of a particular UE, causing degradation of reception performance. In order to address this problem, transmission power of the uplink may be increased, but also serves as interference to other uplink signals, causing deterioration of reception performance. Due to these problems, there is a limitation on the received power level of the uplink signals, at which Node B can receive the uplink signals, guaranteeing the reception performance. This will be described using Rise-Over-Thermal (ROT) defined asROT=Io/No   (1)
In Equation (1), Io denotes the total amount of uplink signals received at Node B, which is the received total broadband power spectral density of Node B, and No denotes thermal noise power spectral density of Node B. Therefore, the allowed maximum ROT can be the radio resource available by Node B in the uplink, that is, the received total wideband power (RTWP) available by Node B in the uplink.
FIG. 2 is a ladder diagram illustrating a basic procedure for transmitting/receiving an E-DCH.
Referring to FIG. 2, the communication system includes a UE 202 receiving an E-DCH and a Node B 201 to which the UE 202 belongs.
In step 203, E-DCH setup between Node B 201 and the UE 202 is achieved for transmission/reception of an E-DCH. The setup process includes a process of delivering messages over a dedicated transport channel. In step 204, the UE 202 provides scheduling information to Node B 201. The scheduling information can include UE's transmission power information based on which uplink channel information can be found, and can also include information on the residual power transmittable by the UE, or information on the amount of transmission data piled up in a buffer of the UE. Upon receiving the scheduling information from several UEs, Node B 201 schedules each of the UEs while monitoring the scheduling information from several UEs in step 211.
If Node B 201 determines to perform scheduling for allowing the UE 202 to transmit uplink packets, Node B 201 transmits scheduling assignment information to the UE 202 in step 205. Then the UE 202 determines a transport format (TF) of the E-DCH to be transmitted in the uplink direction using the scheduling assignment information in step 212, and transmits the TF information and the E-DCH to Node B 201 in steps 206 and 207.
Upon receiving the E-DCH, Node B 201 determines in step 213 whether there is any error in the received TF information and E-DCH. In step 208, the Node B 201 transmits Negative Acknowledgement (NACK) information to the UE 202 if there is an error, and transmits Acknowledgement (ACK) information to the UE 202 if there is no error.
If the ACK information is transmitted in step 208, transmission of the E-DCH information in step 207 is terminated so the UE 202 can transmit new information over the E-DCH. However, if the NACK information is transmitted in step 208, the UE 202 retransmits the same information over the E-DCH.
Next, a description will be made of a scheduling method performed by Node B 201. The scheduling method can be roughly classified into a rate scheduling method and a time and rate scheduling method.
In the rate scheduling method, a Node B increases/keeps/decreases a data rate by a predetermined level every scheduling period for all UEs requiring E-DCH service. That is, in the system where the TF is set such that a UE may have data rates of 16, 32, 128, 256, 384, and 568 kbps, and a Node B increases/keeps/decreases the data rate step by step, if the currently allocated maximum data rate is 16 kbps and Node B issues an Up (or Increase) command in the next scheduling period, the maximum allowable data rate is increased from 16 kbps to 23 kbps by one level. The rate scheduling method, which schedules many UEs, may bring a signaling overhead when the amount of signaling information transmitted every time is large. Therefore, the rate scheduling method transmits the scheduling information using a relative grant. In the relative grant-based scheduling method, if Node B signals limited information such as +1/0/−1, the UE receiving the information increases/keeps/decreases the currently set maximum allowable data rate by a predetermined level depending on the received information. The relative grant is transmitted over an Enhanced Relative Grant Channel (E-RGCH).
The foregoing rate scheduling method can reduce the signaling overhead of the downlink because the required amount of transmission information is small. However, to abruptly increase the data rate, the method requires a long time. Because the relative grant requires 1 bit, there is a possible alternative scheduling method for defining a unique transmission time for each UE in one shared channel on a time multiplexing basis, or allocating a unique orthogonal code to each UE.
The time and rate scheduling method, which controls even the time that the UE transmits the E-DCH, can only schedule some of multiple UEs and abruptly increase or decrease the data rate. The time and rate scheduling method delivers information using an absolute grant. In the absolute grant-based scheduling method, Node B transmits a value of the maximum data rate scheduled to a desired UE, and the UE receiving the value sets the maximum allowable data rate based on the received information.
For example, if the UE has the maximum allowable data rate of 16 kbps and has a lot of data to transmit, Node B can allocate a corresponding data rate such that the UE can transmit the data at up to 568 kbps in the next scheduling period.
In the time and rate scheduling method, Node B should be aware of the maximum allowable data rate of the UE. The maximum allowable data rate is determined according to a TF set value allocated to the UE. This is called a ‘Node-B pointer’.
Because the time and rate scheduling method increases in the required amount of information to provide the absolute grant to the UE as described above, if each individual UE uses its own dedicated channel, transmission power of the downlink increases. Therefore, HSDPA transmits the absolute grant using a shared channel such as High Speed Shared Control Channel (HS-SCCH), and can transmit a UE identifier (UE-id) together to indicate that the absolute grant is information signaled to the corresponding UE. The channel used for transmitting the absolute grant is called an Enhanced Shared Control Channel (E-SCCH).
Because both of the foregoing two scheduling methods have merits and demerits, the uplink packet transmission system needs to be designed such that it supports both the rate scheduling method and the time and rate scheduling method to meet the delay requirements of the UEs, contributing to a reduction in the signaling overhead.
Next, a description will be made of a method for transmitting an Enhanced Absolute Grant Channel (E-AGCH) used for transmitting the absolute grant. The E-AGCH contains the absolute grant and is transmitted over a shared channel because there is no need for all UEs in a cell to perform scheduling in the absolute grant-based scheduling method every Transmit Time Interval (TTI). A UE-id is allocated in the E-AGCH to indicate that the E-AGCH is information signaled to the corresponding UE, and the UE performs Cyclic Redundancy Check (CRC) thereon. If the UE succeeds in the CRC, the UE transmits an E-DCH using corresponding information, determining that the corresponding information is transmitted thereto.
A description will now be made of an operation in a soft handover (SHO) state in which several Node Bs exits in the system using both the absolute grant and the relative grant.
The absolute grant-based scheduling method is higher in channel decoding complexity than the relative grant-based scheduling method, because the E-AGCH contains a lot of information and uses high power. Therefore, receiving the absolute grant only from one Node B is preferable. In this instance, the one Node B transmitting the absolute grant is called a ‘serving Node B’, and a Node B having the best downlink is selected as the serving Node B in a predetermined procedure or HSDPA. That is, a UE in the SHO state receives both the absolute grant and the relative grant from the serving Node B among several Node Bs, and receives only the relative grant from the other Node Bs, that is, non-serving Node Bs, except for the serving Node B.
Instead of signaling all of up/down/keep (or increment/decrement/keep) used in the general relative grant-based scheduling method, the non-serving Node B, which has no scheduling right for the corresponding UE, commands the UE to decrease the data rate (Down Command) taking into account the ROT conditions of the current cell if a ratio of ROT of other UEs in the SHO state is high, and otherwise, transmits no signal (Don't Care) to allow each of the UEs to follow scheduling of its own serving Node B. The information used for controlling the data rate in this manner is called an ‘overload indicator’. In the foregoing case, it is possible to either signal the overload indicator to all UEs on a dedicated basis, or signal the overload indicator to all the UEs on a common basis taking the downlink signaling overhead into account.
FIG. 3 is a diagram illustrating a SHO state in which one UE receives scheduling information from several Node Bs.
Referring to FIG. 3, the number of cells (cell#1 to cell#5) included in an active set of a UE 301 is five, and the five cells include two cells 321 and 322 managed by a Node B1 311, two cells 323 and 324 managed by a Node B2 312, and a cell 325 managed by a Node B3 313.
The cell#1 321 is set as a serving cell that can transmit an absolute grant to the UE 301, and the UE 301 receives an E-AGCH and an E-RGCH from the serving cell 321. However, the UE 301 receives only the E-RGCH, a relative grant transport channel, from the cell#2 322 to the cell#5 325 except for the serving cell 321. Different cells managed by one Node B can transmit the same relative grant for one UE. That is, a set of radio links managed by a Node B among several radio links where one UE 301 exits (one radio link exists in one cell for one UE) is called a ‘radio link set (RLS)’. In FIG. 3, because the number of Node Bs participating in the soft handover is three, there are three RLSs 331, 332 and 333. In an exemplary implementation, an RLS related to a serving Node B will be referred to as a “serving RLS,” and an RLS related to a non-serving Node B will be referred to as a “non-serving RLS.”
When a UE consecutively receives Down commands from different RLSs in a SHO state, the current WCDMA system uses a method of setting a timer each time the WCDMA issues a Down command so that the UE no longer decreases its data rate, even though the UE receives another Down command within a predetermined time, in order to prevent the data rate from abruptly decreasing. This method is generally called a ‘hysteresis method’, and the predetermined time is called a ‘hysteresis period’.
FIG. 4 is a diagram illustrating an exemplary operation of a UE in the SHO state of FIG. 3. In FIG. 4, X represents “keep” for a serving RLS, and “don't care” for a non-serving RLS. In addition, it is assumed that scheduling commands are received over an E-RGCH every 10 ms, and a hysteresis period is set to 20 ms.
Referring to FIG. 4, scheduling commands on the E-RGCH are transmitted from an RLS2 420 and an RLS3 430, both which are non-serving RLSs. Each time the UE receives Down commands 421 to 424 and 431 to 433 among the scheduling commands, the UE checks whether it has ever reduced its data rate for their previous hysteresis periods 451 to 457. The UE decreases its data rate according to the received commands if it has decreased the data rate. Otherwise, the UE keeps the current data rate.
Specifically, when receiving Down commands 421, 422 and 423, the UE decreases its data rate according to the received commands at times 461 to 463 because the UE has decreased the data rate for their previous hysteresis periods 451, 453 and 456. However, when receiving the other Down commands 424, 431, 432 and 433, the UE keeps its current data rate at times 461 to 463 because it has decreased the data rate for their previous hysteresis periods.
However, in this data rate decreasing method, when the UE receives consecutive Down commands (423, 424) and (432, 433) from one RLS, because the UE cannot consecutively receive Down commands, uplink radio resources of the corresponding cell that transmitted the Down commands may continue in the overload state.